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Community and Q&A

Air-sealing a metal chimney

User avatar
Benjamin Pries | Posted in Green Building Techniques on

Hello GBA community, I have a question about air-sealing an ICC Excel chimney servicing a woodstove in Manitoba, Canada:

I am installing the chimney using a square cathedral support which doubles as an attic radiation shield. I have already purchased the support and chimney components, and am ready to install the chimney. It is going into a cathedral ceiling insulated with dense-pack cellulose. The roof will be vented with the vent channel outboard of the roof sheathing. The cathedral support passes through both air barriers: ice & water shield on top of the ceiling boards at the interior, and ZIP-taped plywood sheathing at the exterior.

I phoned ICC this morning to get some guidance on what sealants or tapes to put in contact with the cathedral support, and was told that despite having zero clearance to combustibles required (i.e. wood framing and non-foam insulation in contact with the cathedral support is acceptable), no tape or sealant of any kind can be used. The support person with whom I spoke acknowledged the existence of fire-rated caulks but was adamant that there was no possible way for me to maintain the continuity of my air barrier using the conventional cathedral support that I have.

She advised me that ICC makes an insulated support with a flange meant to seal to an interior membrane, which can only be installed to a level surface such as a flat ceiling or built-out box, and encouraged me to refer to the website, which says, “The Insulated Round Support (ERDSI) is designed for installations in energy efficient houses which have tightly sealed vapour barriers. The RDSI is fully insulated and has a flange to permit an air tight seal to the vapour barrier.” She also told me that if it was possible to air-seal the other supports, the product description would have explicitly stated so.

Is that true? I can’t believe that the typical support used in the vast majority of chimney installations would be completely incompatible with any attempt at air-sealing, and I feel like the support person I talked to was out to lunch, based on other descriptions at GBA and elsewhere. If she’s right, what are my options? I live in the sticks, and any ICC product is a special-order item that I can’t return, so I would need to eat the square support and buy this second insulated support in its place. Alternatively, should I be building a plywood chase around the support, and then air-sealing that so that the air leaking around the cathedral support leaks straight up to the vent channel and at least doesn’t infiltrate the insulated cavity? Lastly, if she is wrong, and I can apply some sealants or tapes to the cathedral support, what can I use that will also make a durable seal to the granular surface of the ice-and-water shield?

As usual, I’ve taken a lot of words for a small problem. Thanks in advance for your time!

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    First of all, you don't want any cellulose to come in contact with the chimney. The portion of the chimney that penetrates the insulation layer needs to be wrapped in a mineral wool batt secured in place with steel wire.

    Second, I feel confident in asserting that as long as all required clearances between the chimney and combustible materials (as described in the installation instructions provided by the manufacturer) are maintained, that it is acceptable to install sheet metal adjacent to the chimney penetration (typically this consists of two overlapping pieces of sheet metal, each with a semi-circular cutout to accommodate the chimney), and to caulk the crack between the sheet metal and the chimney with high-temperature silicone caulk.

    The above recommendations apply to a double-walled chimney, of course, not to stovepipe.

    My advice may or may not comply with recommendations by ICC. But others have followed this path, and I've never heard of problems.

  2. User avatar
    Benjamin Pries | | #2

    Thanks Martin. There will definitely not be cellulose in contact with the chimney. The support maintains a 1 inch airspace between the outside of the insulated chimney and everything outside of the support. It is essentially a sheet metal box that gets mounted into the opening through which the chimney passes. It has a round hole at the bottom through which the bottom of the insulated chimney sticks to meet the stovepipe coming from inside. At the top, my impression is that it is typically closed off with a storm collar or sheet metal as you described.

    As far as I can see, the installation manual makes no mention of limitations on materials in contact with the outside of the cathedral support. It does however, reiterate the suggestion of the specialized support where the builder is aiming for an airtight installation: "If you are installing your chimney in an energy efficient house which has sealed vapour barriers (e.g.: R2000 construction) we manufacture a special round support so that you can maintain your vapour barrier integrity."

    I suppose what I will do is seal the wood framing of the opening to the ice and water shield with sealant or tape and use hi-temp silicone to seal the support to the framing, and the same again to seal the chimney to the support where they meet at the bottom. Then I will close off the top with sheet metal sealed to the chimney with the hi-temp silicone again, and seal that metal closure to the plywood sheathing with the same hi-temp silicone. Hopefully the house burns down in a forest fire before the woodstove installation has a chance to go wrong...

  3. User avatar
    Rob Myers | | #3

    Hi Benjamin,
    I think that it is always best to follow the installation guide and if you are deviating in any way from the printed instructions then you should confirm with Excel that your procedure is OK. A chimney is probably not a suitable subject for experimentation.
    I am not an expert but I have installed the ERDSI chimney support in my house and the main advantage of this support in energy efficient houses is that it is insulated between the chimney and the radiation shield (the assembly is tested and cannot be modified).
    I very seldom disagree with Martin (and perhaps I misunderstood his description) but you cannot wrap insulation around an Excel metal chimney - it is a high performance assembly that has a thinner layer of insulation in the chimney itself (I believe it is 1" rather than 2"). My understanding is that nothing can be inserted into the air gap between the chimney and the radiation shield and I'm not sure that you are allowed to seal the top of the radiation shield the way that you described.
    There is no particular problem with air sealing the regular support from the inside as described in the manual, but the support will not be insulated.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    I agree that no insulation should be inserted between the chimney and the radiation shield. I was proposing details for insulated metal chimneys that penetrate insulation layers where radiation shields are not required by the chimney manufacturer.

    The main point I intended to convey is that cellulose insulation should never touch a chimney. In any location where insulation may touch the exterior of a metal chimney, mineral wool is safer than cellulose.

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